A joint within the human body forms a juncture between two or more bones or other skeletal parts. The ankle, hip, knee, shoulder, elbow and wrist are just a few examples of the multitude of joints found within the body. As should be apparent from the above list of examples of joints, many of the joints permit relative motion between the bones. For example, the motion of sliding, gliding, hinge or ball and socket movements may be had by a joint. For example, the ankle permits a hinge movement, the knee allows for a combination of gliding and hinge movements and the shoulder and hip permit movement through a ball and socket arrangement.
The joints in the body are stressed or can be damaged in a variety of ways. For example, gradual wear and tear is imposed on the joints through the continuous use of a joint over the years. The joints that permit motion have cartilage positioned between the bones providing lubrication to the motion and also absorbing some of the forces direct to the joint. Over time, the normal use of a joint may wear down the cartilage and bring the moving bones in direct contact with each other. In contrast, in normal use, a trauma to a joint, such as the delivery of a large force, from an accident, for example, an automobile accident, may cause considerable damage to the bones, the cartilage or to other connective tissue such as tendons or ligaments.
Arthropathy, a term referring to a disease of the joint, is another way in which a joint may become damaged. Perhaps the best known joint disease is arthritis, which is generally referred to a disease or inflammation of a joint that results in pain, swelling, stiffness, instability, and often deformity.
There are many different forms of arthritis, with osteoarthritis being the most common and resulting from the wear and tear of a cartilage within a joint. Another type of arthritis is osteonecrosis, which is caused by the death of a part of the bone due to loss of blood supply. Other types of arthritis are caused by trauma to the joint while others, such as rheumatoid arthritis, Lupus, and psoriatic arthritis destroy cartilage and are associated with the inflammation of the joint lining
The hip joint is one of the joints that is commonly afflicted with arthropathy. The hip joint is a ball and socket joint that joins the femur or thighbone with the pelvis. The pelvis has a semispherical socket called the acetabulum for receiving a ball socket head in the femur. Both the head of the femur and the acetabulum are coated with cartilage for allowing the femur to move easily within the pelvis. Other joints commonly afflicted with arthropathy include the spine, knee, shoulder, carpals, metacarpals, and phalanges of the hand. Arthroplasty as opposed to arthropathy commonly refers to the making of artificial joint. In severe cases of arthritis or other forms of arthropathy, such as when pain is overwhelming or when a joint has a limited range of mobility, a partial or total replacement of the joint with an artificial joint may be justified. The procedure for replacing the joint varies, of course, with the particular joint in question, but in general involves replacing a terminal portion of an afflicted bone with a prosthetic implant and inserting a member to serve as a substitute for the cartilage.
The prosthetic implant is formed of a rigid material that becomes bonded with the bone and provides strength and rigidity to the joint and the cartilage substitute members chosen to provide lubrication to the joint and to absorb some of the compressive forces. Suitable materials for the implant include metals, and composite materials such as titanium, cobalt chromium, stainless steel, ceramic and suitable materials for cartilage substitutes include polyethylene. A cement may also be used to secure the prosthetic implant to the host bone.
A total hip replacement, for example, involves removing the ball shaped head of the femur and inserting a stem implant into the center of the bone which is referred to as the medullary canal or marrow of the bone. The stem implant may be cemented into the medullary canal or may have a porous coated surface for allowing the bone to heal directly to the implant. The stem implant has a neck and a ball shaped head which are intended to perform the same functions as a healthy femur's neck and a ball shaped head. The polyethylene cup is inserted into the acetabulum and has a socket for receiving the head on the stem implant.
One challenge in the proper positioning of the prosthesis during surgery is the proper position of the stem axially and rotationally. Improper positioning has been shown to limit the patient's range of motion by inducing improper leg length, inadequate lateral stem offset and non-anatomical version of the stem. Inadequate pressurization of the cement within the femoral canal has also been documented as a potential cause of improper cement technique.
Centralization of the stem within the cement mantle is also critical for success. Non-uniform or excessively thin cement mantles can induce high cement stress and subsequent cracks that may cause failure at the cement-stem-bone interfaces. The cement debris, due to abrasions, has also been shown to produce excessive third-body wear of polyethylene acetabular components as well as potentially induce osteolytic reactions and bone resorptions that may lead to stem loosening.
One device utilized to assist in the centralization of the stem is a centralizer or spacer. Centralizers or spacers are provided for fitting to the distal end of a femoral hip replacement stem in order to keep the implant stem away from the internal surface of the cavity of the bone in which this stem is to be inserted.
In the case of stems which are cemented in the bone cavity there is a space between the stem and the internal surface of the cavity of the bone in which the cement is placed. Controlling the position of the stem within the surrounding bone cement mantle is vital to long-term survivability of the replacement joint. Cement can be deposited in the bone cavity and then the stem may be inserted with the centralizer attached to the stem. Alternatively, the centralizer may be inserted into the cavity and the stem later inserted against the centralizer. It is important to try to obtain an even and intact cement mantle around the stem.
In addition to the purpose of the centralizer to properly position the stem, the centralizer may be designed to serve a second purpose, that is to separate the cement from the blood and other body fluids within the medullary canal of the bone. Such separation of cement and medullary canal fluids is exasperated by the more recent use of external pressure to assure the complete filling of the bone cavity with cement.
Known centralizers are in the form of caps which fit over the distal end of the stem and centralizers which are fixed inside of a drilled end of a stem. Centralizers are also known which are of ring form which can have a tapered inner surface corresponding to the tapered surface of the distal stem of the femoral stem on which the centralizer is located.
Centralizers or spacers in the form of a cap for insertion on the end of a hip stem with fins or wings extending outwardly from the cap which are adapted to fold circumferentially and inwardly toward the body portion of the cap.
Cemented stem systems generally utilize two components distal to the stem, a centralizer and a cement plug. The centralizers usually have fins that protrude into the cement mantle around the stem.
Due to variations in the age, gender, and size of a patient, a wide variety of distal centralizers are available for use in the medullary canal of a patient. The proper or optimum size from the available variety of sizes of the centralizer must be selected by the surgeon. When implanting a cemented stem, a surgeon has to make an educated guess using for example x-rays and templating to determine the proper sized distal centralizer to use.
Due to the two-dimensional nature of x-rays and templating, and the three-dimensional shape of the centralizer, such techniques are crude and inaccurate. Such techniques may require the removal of the first chosen centralizer and a second centralizer used or may result in a less than ideal centralizer being utilized.
Attempts have been made to provide for a more accurate way of determining the proper size for centralizer for the surgeon to use on a particular patient. For example, a canal sizer has been developed by Zimmer Holding, Inc., Warsaw, Ind.; sold as the IM Sizer, which provides for a plug having a round cross-section. This device may have a tendency to get caught in the distal canal before its proper position is reached. Also, this device is adapted for only one particular brand and style of orthopedic implant.
The present invention is adapted to overcome at least one of the aforementioned shortcomings of the prior art.